The plant life cycle alternates between a diploid and a haploid generation, the sporophyte and the gametophyte. Unlike in animals where meiotic products differentiate directly into gametes, the plant spores undergo several divisions to form a multicellular organism. Differentiation of the gametes occurs later in gametophyte development.
Reproduction in plants is ordinarily classified as sexual or asexual. The term apomixis is generally accepted as the replacement of sexual reproduction by various forms of asexual reproduction (Rieger et al., IN Glossary of Genetics and Cytogenetics, Springer-Verlag, New York, N.Y., 1976). In general the initiation of cell proliferation in the embryo and endosperm are uncoupled from fertilization. Apomixis is a genetically controlled method of reproduction in plants where the embryo is formed without union of an egg and a sperm. There are three basic types of apomictic reproduction: 1) apospory-embryo develops from a chromosomally unreduced egg in an embryo sac derived from a somatic cell in the nucellus, 2) diplospory-embryo develops from an unreduced egg in an embryo sac derived from the megaspore mother cell, and 3) adventitious embryony-embryo develops directly from a somatic cell. In most forms of apomixis, pseudogamy or fertilization of the polar nuclei to produce endosperm is necessary for seed viability. These types of apomixis have economic potential because they can cause any genotype, regardless of how heterozygous, to breed true. It is a reproductive process that bypasses female meiosis and syngamy to produce embryos genetically identical to the maternal parent. With apomictic reproduction, progeny of specially adaptive or hybrid genotypes would maintain their genetic fidelity throughout repeated life cycles. In addition to fixing hybrid vigor, apomixis can make possible commercial hybrid production in crops where efficient male sterility or fertility restoration systems for producing hybrids are not known or developed. Apomixis can make hybrid development more efficient. It also simplifies hybrid production and increases genetic diversity in plant species with good male sterility.
In sexual reproduction, usually a megaspore mother cell arising from the hypodermal layer of the ovule enlarges and goes through meiosis and two cell divisions to form a linear tetrad of megaspores each with a haploid chromosome number. The three micropylar spores degenerate while the functional chalazal spore enlarges to form an embryo sac with an egg, two polar nuclei, two synergids, and three antipodals.
Introducing the apomictic trait into normally sexual crops has been attempted. Asker (Heredias, Vol. 91, 231-241, 1979) reports that attempts have been unsuccessful with wheat, sugar beets, and maize. PCT publication WO 89/00810 (Maxon et al, 1989) discloses inducing an apomictic form of reproduction in cultivated plants using extracts from nondomesticated sterile alfalfa plants. When induction of male sterility was evaluated in sorghum, sunflower, pearl millet, and tomato it was reported that there was reduced seed set in sorghum, pearl millet, and sunflower and reduced fruit set in tomato.
It would be ideal to find genes controlling obligate or a high level of apomixis in the cultivated species and be able to readily hybridize cross-compatible sexual x apomictic genotypes to produce true-breeding F.sub.1 hybrids. In reality, most desirable genes controlling apomixis are found in the wild species which are distantly related to the cultivated species. Although interspecific crosses may be possible between the cultivated and wild species, chromosome pairing between genomes is usually low or nonexistent.
Although apomixis is effectively used in Citrus to produce uniform and disease- and virus-free rootstock (Parlevliet JE et al, in Citrus. Proc. Am. Soc. Hort. Sci., Vol. 74, 252-260, 1959) and in buffelgrass (Bashaw, Crop Science, Vol. 20, 112, 1980) and Poa (Pepin et al, Crop Science, Vol. 11, 445-448, 1971) to produce improved cultivars, it has not been successfully transferred to a cultivated crop plant. The transfer of apomixis to important crops would make possible development of true-breeding hybrids and commercial production of hybrids without a need for cytoplasmic-nuclear male sterility and high cost, labor-intensive production processes. An obligately apomictic F.sub.1 hybrid would breed true through the seed indefinitely and could be considered a vegetative or clonal method of reproduction through the seed. The development of apomictically reproducing cultivated crops would also provide a major contribution toward the food security in developing nations (Wilson et al, IN Proceedings of the International Workshop on Apomixis in Rice, Changsha, People's Republic of China, Jan. 13-Jan. 15, 1992. Hunan Hybrid Rice Research Center, Changsha, People's Republic of China).
The generation of apomixis is only one of the many potential benefits of controlling cell proliferation in the embryo or endosperm portions of the seed. Induction of parthenocarpy for seedless fruits and vegetables or the production of value added custom seeds which involve enhancement of certain tissue areas at the expense of others is another. As can be seen from the foregoing, there is a need in the art for tissue specific control of cell proliferation.
It is thus an object of the present invention to provide a novel gene and protein which regulate cell proliferation in a tissue specific manner.
It is yet another object of the invention to provide a DNA sequence which encodes a gene from the Polycomb family from Arabidopsis which is involved with initiation and control of embryo and endosperm cell proliferation.
A further object is to provide a mechanism to manipulate embryo and endosperm cell proliferation to achieve apomixis or fertilization independent cell proliferation.
A further object of the present invention is to provide constructs for expression of or inhibition of this gene product.
A further object of the invention is to provide a method for controlling undesirable seed production through the use of a suicide gene.
Finally, it is an object of the present invention to provide genetic material which can used to screen other genomes to identify other genes with similar effects from other plant sources or even from animal sources.